The present invention relates to an optical semiconductor device in which an optical semiconductor element is bonded with and fixed to an optical circuit substrate, and a manufacturing method of the same
An example of an optical semiconductor device in which an optical semiconductor element is joined with an optical circuit substrate, is proposed in the pending U.S. patent application Ser. No. 08/126,769, entitled "Optical coupling equipment for An Optical Semiconductor and An Optical Fiber" by the inventors of the present patent application.
In order to join the optical semiconductor element with the optical circuit substrate, gold-tin (AuSn) eutectic solder is generally used. More particularly, the solder is first melted by heating and at this time the optical semiconductor element is placed on the optical circuit substrate to be pressed to the optical circuit substrate so that the optical semiconductor element can be joined on the optical circuit substrate.
However, because the AuSn eutectic solder is easily oxidized, the joining process must be performed in nitrogen atmosphere or reduction atmosphere in which hydrogen gas is slightly mixed with nitrogen gas. In addition, before the optical semiconductor element is pressed to the optical circuit substrate, a so-called "scrubbing process" needs to be performed in which the optical semiconductor element is applied with a pressure and is vibrated for removing an oxide film formed on the AuSn eutectic solder in advance.
As another bonding method, there is known the method in which Sn has been deposited on the junction area of an electrode of the optical circuit substrate or the junction area of the optical semiconductor element, and both the optical circuit substrate and the optical semiconductor element are heated and pressed to each other to form the junction. In this case, because Sn is easily oxidized, the non-oxidization atmosphere and the scrubbing process for the optical semiconductor element are required.
Further, there is another method other than the above methods in which a layer of AuSn eutectic solder is formed on a junction portion of the optical circuit substrate, a thin Au layer is formed on the layer of eutectic solder. An optical semiconductor element is placed on the Au layer so that the junction is formed by melting the layer of eutectic solder. The detail of this method is disclosed in Japanese Patent Application Laid-Open TokuKaiHei-1-138777 (1989).
There have been many proposals of the structure in which optical coupling to an optical fiber can be performed without optical axis alignment. For achieving the structure, the optical semiconductor element needs to be bonded with the optical circuit substrate at a predetermined position with a high precision. Even if the optical semiconductor element is to be positioned and bonded with a high precision of submicron order by use of the above conventional bonding methods, the junction position shifts because the optical semiconductor element is vibrated in the above scrubbing process.
A melt of AuSn eutectic solder is 20 to 30 .mu.m in thickness. Therefore, when the optical semiconductor element is made to contact with the melted AuSn eutectic solder, the surface of an electrode layer of the optical semiconductor element is wet and at the same time the optical semiconductor element is randomly moved because of surface tension of the AuSn eutectic solder. Here, that "the electrode layer surface is wet" means that the melted AuSn eutectic solder extends thinly on the electrode layer surface. Further, because the optical semiconductor element falls into the melted AuSn eutectic solder by 10 to 20 .mu.m by application of pressure, the positioning of the optical semiconductor element with the submicron order is very difficult in the conventional bonding methods.
Even if Sn is deposited, the scrubbing process is required because an oxide film is produced on the layer of eutectic solder like in the above joining methods. For this reason, it is impossible to position the optical semiconductor element at a predetermined position with a high precision of submicron order. In addition, it is difficult to produce a perfect AuSn eutectic layer only by diffusion of Sn and Au in bonding of the optical semiconductor element. In this case, a weight % ratio of Sn to Au becomes so great that a fragile metallic compound is easy to produce. Therefore, it is difficult to ensure reliability after bonding. Hence, it is necessary to strictly control the storage condition, a temperature in the bonding and a heating period.
Further, even in a case that a thin Au layer is deposited on an AuSn eutectic solder layer formed on a junction portion of the optical circuit substrate, it is necessary to make the Au layer thin for keeping the eutectic condition. Consequently, the scrubbing process would be required as in the above methods because Au of the Au layer and Sn of the AuSn eutectic solder layer diffuses mutually into the eutectic layer and the Au layer and as a result of this an oxide film is produced on the surface of the junction portion. In this case, if the deposited layer of Au is made so thick that the oxide film can be prevented from being producing, another problem is caused in which the eutectic condition cannot be kept because of the mutual diffusion of Au of the Au layer and Sn of the AuSn eutectic solder layer. In addition, it is often difficult to keep the stable bonded state after the melting and bonding when the peripheral temperature of the junction portion is raised, more particularly, in a high temperature because a melting point of the AuSn eutectic state is low.